In recent years, light weight transparent polymer films have been extensively used in the field of displays exemplified by cellular phone displays. Polymer films are also considered to be one of the essential components for a so-called “paper-type display”—a product of emerging technology. Polymers in the form of films which are most suitable for use in various fields include polyethylene, polypropylene, polyethylene terephthalate, or similar crystalline polymer, and polycarbonate, polymethylmethacrylate, or similar amorphous polymer. All these materials are thermoplastic polymers that can be easily formed into films of various types by adjusting the molecular weight and molecular-weight distribution. At the present time, a great variety of highly transparent polymer films available on the market are made from thermoplastic polymers. For example, such films are produced by subjecting a hot-molten thermoplastic polymer to calender treatment, or by extruding the polymer through a T-die. It is also known to produce transparent polymer films by subjecting crystalline polymers to biaxial stretching.
It is known that orientation of high-molecular chains in films made from thermoplastic polymers presents a problem since such an orientation is inherent in the methods themselves used for the manufacture of such films. For example, the molecular chains of a hot-molten polymer are oriented under the effect of calender rollers when the thermoplastic polymer is subjected to calendar treatment or under the effect of extrusion forces when the aforementioned polymer is extruded through a T-die in an extrusion process. Furthermore, stretching of a polymer film is always accompanied by orientation of the polymer chains in the stretching direction. However, the aforementioned phenomenon of molecular chain orientation may present a problem for transparent films. This is because orientation of polymer chains in a transparent film generates birefringence. Therefore, practical application of transparent films of thermoplastic polymers as an optical material presents difficulties. A source of orientation of macromolecules is the stress applied to the material in a hot-molten state. However, during molding in a mold, application of a certain stress to a hot-molten material is inevitable. In order to suppress molecular orientation in films, such methods as cast molding were developed. Nevertheless, manufacture of thermoplastic polymer films, even by cast molding, still suffers from a lot of problems. For example, evaporation of unreacted monomers of thermoplastic polymers, solvents used in cast molding, and various additives introduced, e.g., for imparting heat-resistant properties, from the surfaces of the films contaminates the surrounding environment. To prevent unreacted monomers from evaporation, the synthesized thermoplastic polymers could be purified before obtaining the cast molding solution, but evaporation of the solvent can not be avoided. Furthermore, such processes require additional costs and production time. Besides, films produced from thermoplastic polymers normally have low resistance to heat and are rarely used for manufacture of electronic parts that operate under heat-generating conditions since at high temperatures such parts lose their mechanical strength. Known in the art are thermoplastic amorphous polymers with high heat-resistant properties, such as polysulfone. However, polysulfone has light absorption in the vicinity of 400 nm, and therefore cannot be efficiently used as an optical material in view of its light transmittance characteristics.
Let us now consider the formation of films from thermosetting resins, e.g., from cross-linkable polymers that possess excellent heat-resistant properties. In the case when a cross-linkable polymer is used, a liquid monomer or a low-molecular-weight prepolymer acquires a predetermined shape by being cross-linked and therefore does not require application of stress and is not subject to molecular orientation. Furthermore, if a film made from a cross-linkable polymer such as a thermosetting resin contains residual monomers, e.g., a low-molecular-weight compound, the latter is captured into the three-dimensional polymer network and diffusion from the film is suppressed. Therefore, manufacture of such films is free of problems inherent in mold-casting from the aforementioned thermoplastic resins.
In a majority of cases, however, the films made from cross-linkable polymers such as thermosetting resins are provided in a state held on a prescribed substrate or coated on a prescribed substrate. It was difficult to manufacture any free-standing film made of cross-linkable polymers that can be used without being held on a substrate. In particular, it is possible to manufacture transparent films made from curable organopolysiloxane compositions having excellent resistance to heat, resistance to ultraviolet radiation, anti-oxidation properties, etc., without addition of heat-resistant agents, UV absorbants, or antioxidants. However, such films are held on substrates, and free-standing films, i.e., unsupported films made of cured organopolysiloxanes that possess physical properties sufficient for practical use are still absent from the market. The inventors herein developed a free-standing film that is characterized by high heat-resistant properties, excellent transparency in the visible-light range, and low birefringence. The inventors also developed layered films having an inorganic layer on the aforementioned free-standing film made of cured organopolysiloxanes. These films are disclosed in International Patent Application Publication WO 03/104329.
However, WO 03/104329 does not disclose a free-standing film comprising a cured organopolysiloxane that has a high deformation temperature, i.e., glass-transition temperature that can withstand high temperatures occurring in the below-described processes for forming an inorganic-compound layer, and that has a small coefficient of thermal expansion. For example, in the manufacture of displays from the aforementioned films, the inventors encountered a problem of mismatch between a thermal expansion coefficient of the substrate film and a thermal expansion coefficient of a vapor-deposited layer.